The invention relates to a method and a device for replacing equipment on the seabed.
Equipment which is placed on the seabed in connection with extraction of hydrocarbons from deposits found at sea may require replacement or retrieval for maintenance purposes or because the equipment fails after a period of time.
For replacement, the equipment has to be brought up to the surface. For this purpose a pipe string or wires/chains may be employed. If the object which has to be retrieved is very heavy, a pipe string is normally used. In this case a rig or drilling vessel with a derrick is employed. However, vessels of this kind are extremely expensive. If it is possible, therefore, the use is preferred of smaller vessels equipped with a crane. The equipment is then lowered by means of a wire, chain or the like.
A disadvantage of the present methods is that heave compensator systems have to be used. A vessel floating on the water will be exposed to wind an weather as well as wave motion. Waves cause the vessel to move vertically relative to the seabed, which may result in damage to the equipment during the operation. Heave compensator systems compensate for heave or wave motion. Such systems therefore ensure that the pipe string or the wire, and thereby the equipment, are kept reasonably stationary relative to the seabed. There are two main types of heave compensator systems, an active type and a passive type.
A second disadvantage of the present methods is that replacement of equipment normally has to be undertaken in several stages. Firstly, the retrieval device has to be lowered to get hold of the equipment which is to be replaced. This has to be brought up to the surface, whereupon the new equipment can be lowered and placed in position. This takes time and therefore is costly.
From U.S. Pat. No. 4,167,215 a device is known for replacing equipment on the seabed. The device comprises a store for storing equipment. This means that the device can take new equipment down, thus enabling it to be replaced by the equipment on the installation without the need for several trips up and down. However, the device is complicated and is dependent on accurate positioning or alignment of the device relative to the installation on the seabed. The seabed installation also has to be prepared for this by having receiving and orientating bodies for the device.